It is well known that an aqueous solution in contact with an untreated metal substrate can result in corrosion of the untreated metal substrate. Therefore, a metal article, such as a metal container for a water-based product, like a food or beverage, is rendered corrosion resistant in order to retard or eliminate interactions between the water-based product and the metal article. Generally, corrosion resistance is imparted to the metal article, or to a metal substrate in general, by passivating the metal substrate, or by coating the metal substrate with a corrosion-inhibiting coating.
Investigators continually have sought improved coating compositions that reduce or eliminate corrosion of a metal article and that do not adversely affect an aqueous product packaged in the metal article. For example, investigators have sought to improve the imperviousness of the coating in order to prevent corrosion-causing ions, oxygen molecules, and water molecules from contacting and interacting with a metal substrate. Imperviousness can be improved by providing a thicker, more flexible and more adhesive coating, but often, improving one advantageous property is achieved at the expense of a second advantageous property.
In addition, practical considerations limit the thickness, adhesive properties, and flexibility of a coating applied to a metal substrate. For example, thick coatings are expensive, require a longer cure time, can be esthetically unpleasing, and can adversely affect the process of stamping and molding the coated metal substrate into a useful metal article. Similarly, the coating should be sufficiently flexible such that the continuity of the coating is not destroyed during stamping and molding of the metal substrate into the desired shape of the metal article.
Investigators also have sought coatings that possess chemical resistance in addition to corrosion inhibition. A useful coating for the interior of a metal container must be able to withstand the solvating properties of a product packaged in the metal container. If the coating does not possess sufficient chemical resistance, components of the coating can be extracted into the packaged product and adversely affect the product. Even small amounts of extracted coating components can adversely affect sensitive products, like beer, by imparting an off-taste to the product.
Conventionally, organic solvent-based coating compositions were used to provide cured coatings having excellent chemical resistance. Such solvent-based compositions include ingredients that are inherently water insoluble, and thereby effectively resist the solvating properties of water-based products packaged in the metal container. However, because of environmental and toxicological concerns, and in order to comply with increasingly strict governmental regulations, an increasing number of coating compositions are water based. The water-based coating compositions include ingredients that are water soluble or water dispersible, and, therefore, cured coatings resulting from water-based coating compositions often are more susceptible to the solvating properties of water.
In addition, water-based coating compositions do not completely overcome the environmental and toxicological problems associated with organic solvents because water-based compositions typically contain two or more pounds of organic solvent per gallon of coating composition. The organic solvent is a necessary ingredient to dissolve and disperse composition ingredients, and to improve the flow and viscosity of the composition. Therefore, in order to entirely avoid the environmental and toxicological problems associated with organic solvents, investigators have sought solid coating compositions that can be applied to a metal substrate. In attempts to find a useful solid coating composition, investigators have tested powder coatings, laminated film coatings, radiation cure coatings, and extrusion coatings.
Solid powder coatings have been used to coat a metal substrate with a coating composition. Solid coating compositions also have been extruded onto a metal substrate, for example, as disclosed in European Patent No. 0 067 060, PCT publication WO 94/01224, and Smith et al. U.S. Pat. No. 5,407,702.
In both powder coating and extrusion coating applications, an important component of the coating composition is an epoxy resin. Epoxy resins impart several properties that are important with respect to a cured coating composition on a metal substrate. However, epoxy resins also have a serious disadvantage in that residual amounts of bisphenol diglycidyl ether monomer are present in the resin, typically in an amount of about 0.5% by weight. Typical bisphenol diglycidyl ether are bisphenol-A diglycidyl ether (i.e., BADGE) and bisphenol-F diglycidyl ether (i.e., BFDGE). The term "BADGE" as used herein refers to a bisphenol-A diglycidyl ether and to other similar bisphenol diglycidyl ethers, like a bisphenol-F diglycidyl ether. Additional examples of bisphenol diglycidyl ether monomers are illustrated hereafter.
The presence of a BADGE monomer raises serious environmental and toxicological concerns, especially because a BADGE monomer can be extracted from a cured coating on the interior of a metal container by a product stored in the container. Accordingly, regulatory agencies have promulgated regulations reducing the amount of a BADGE monomer in coating compositions, and especially coating compositions used on the interior of food and beverage containers.
In order to overcome the problem of a free BADGE monomer, investigators attempted to use high molecular weight phenoxy resins as a replacement for epoxy resins. Phenoxy resins are essentially very high molecular weight epoxy resins containing very few, if any, intact epoxy rings. This attempted replacement led to other problems, such as difficulty in handling and processing the high molecular weight phenoxy resins in many practical applications, and the very high cost of the phenoxy resins. In addition, coatings containing the phenoxy resins did not provide the excellent adhesion to metal substrates that is provided by coatings containing low-to-medium molecular weight epoxy resins.
Investigators, therefore, have sought solid coating compositions for use on the exterior and interior of food and beverage containers that are free, or essentially free, of a BADGE monomer, that exhibit the advantageous properties of adhesion, flexibility, chemical resistance, and corrosion inhibition, that are economical, and do not adversely affect the taste or other esthetic properties of sensitive foods and beverages packaged in the container. Investigators especially have sought useful solid coating compositions having these desirable benefits and that reduce the environmental and toxicological concerns associated with BADGE monomer. In particular, investigators have sought solid coating compositions for food and beverage containers that (1) meet increasingly strict environmental regulations, (2) have corrosion inhibition properties at least equal to existing epoxy-based and organic solvent-based coating compositions, and (3) are easily applied onto a metal substrate, by powder coating or extrusion techniques, as a thin, uniform film. Such solid coating compositions would satisfy a long felt need in the art.
A present solid coating composition comprises: (a) a base polymer, such as a polyester or blend of polyesters, and (b) an end-capped epoxy resin having an M.sub.w about 300 to about 10,000 and that is essentially free of a BADGE monomer. A present solid coating composition is a thermoplastic composition and can be applied as a powder coating or extruded onto a metal substrate. A crosslinking step, such as an additional heating step after application of the composition onto the metal substrate or use of a crosslinking agent, is not reguired. A present solid coating composition is free of organic solvents, yet an applied film demonstrates excellent coating properties, such as adhesion, hardness, and flexibility.
A solid coating composition of the present invention contains no organic solvents, and, therefore, overcomes the environmental and toxicological problems associated with liquid coating compositions. In addition, a present solid coating composition also avoids the environmental and toxicological problems associated with a BADGE monomer.
The present thermoplastic solid coating compositions provide a sufficiently flexible coating such that the coated metal substrate can be deformed without destroying film continuity. In contrast, thermosetting compositions often provide a rigid cured film thereby making it difficult to impossible to coat the metal substrate prior to deforming, i.e., shaping, the metal substrate into a metal article, like a metal closure, can, can end, can tube, metal drum, or aerosol can. Coating a metal substrate prior to shaping the metal substrate is the present standard industrial practice.
As an added advantage, it is envisioned that a present solid coating composition can be used on can ends, can and drum bodies, can tubes, and closures, thereby obviating the use of different coating compositions by container manufacturers. Furthermore, a present solid coating composition exhibits sufficient clarity, hardness, and mar resistance after application for use as a coating on the exterior of a metal container. Accordingly, an extrusion coating composition of the present invention has a more universal range of applications, such as for the interior coating of a metal container for food or beverage products, or for the exterior coating of a metal container or a material of construction, like aluminum siding, and overcomes the environmental and toxicological concerns associated with epoxy resins and with a liquid coating composition.